Discussion in 'The Projects Forum' started by Lightfire, Mar 20, 2011.

1. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
For example my battery (AA/1.5V) produces 800mAH. My flashlight bulb needs 400mAh to be lighted.

I made a circuit consists of two bulbs (1.5 V/400mAh), battery (AA/1.5/800mAh). It's a parallel circuit.

Do the bulbs' bright is normal? I mean the bulbs' bright is exactly gives????

2. ### Wendy Moderator

Mar 24, 2008
20,993
2,731
This is very close to your other thread. AH and mah are basically the same thing. The m means milli, or 1/1000. So 1 AH equals 1000mah.

Light bulbs are not rated in AH, they are rated in current and voltage. If you have two light bulbs, each pulling 400ma for a total of 800ma, and a battery capable of providing 800mah, the battery will last about 1 hour.

Lightfire likes this.
3. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
So means, the brightness of the two bulbs are OK and normal?

4. ### SgtWookie Expert

Jul 17, 2007
22,194
1,761
Actually, small batteries are rated based on current draw over 20 hours. So, an 800mAh battery is rated for a 40mA load for 20 hours until the battery is considered exhausted (used up, fully discharged).

If a heavier load is placed on the battery, it will become exhausted more quickly due to power dissipation inside the battery itself.

If you put a 400mA load on a battery rated 800mAh, it will not last for 2 hours; maybe an hour or so. If you put an 800mA load on a battery rated for 800mAh, it might last for 20 minutes or so.

5. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,416
902
The AA battery cell in the example is rated at only 800mAh like they were about 50 years ago (super heavy duty, carbon zinc).
Today an AA alkaline cell is rated at 2800mAh at 25mA, 2000mAh at 250mA and 1500mAh at 500mA. But the ratings are when the cell's voltage has dropped to only 0.8V.

6. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
I a little bit agreed at your first post.

But why only 20 minutes on 800mA. It should be one hour, right?

Thanks

7. ### Wendy Moderator

Mar 24, 2008
20,993
2,731
Remember the other thread, where I said the number is meaningless if you overload the battery? If I make you run 4 miles, you won't have any juice left. If I let you walk 4 miles, you will have lots of energy left.

8. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,416
902
Simply go to the website of a battery manufacturer like Energizer or Duracell and look at the datasheet of the battery you want to use.

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9. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
HELLO. ONE more question.

For example my 50 watts/12 volts halogen is pulling or consuming 4.166666667 amps. A 12 volt that produces 12 Ah can run the halogen lamp at about 2.88 hours?

thank you.

Last edited: Mar 22, 2011
10. ### SgtWookie Expert

Jul 17, 2007
22,194
1,761
No, it will be less than that.

If you discharge a lead-acid battery fully, it will not last long at all.

If you operated the lamp from the battery (after fully charging it) for more than about 1 hour, you will likely ruin the battery in a very short period of time.

You really should not discharge a 12v lead-acid battery below ~12v for the longest life span. It should measure about 12.8v when fully charged at 25°C